U.S. patent number 4,738,718 [Application Number 06/792,025] was granted by the patent office on 1988-04-19 for method for the recovery of gold using autoclaving.
This patent grant is currently assigned to Freeport Minerals Company. Invention is credited to Nandkumar Bakshani, Peter H. Yu.
United States Patent |
4,738,718 |
Bakshani , et al. |
April 19, 1988 |
Method for the recovery of gold using autoclaving
Abstract
A gold recovery pretreatment process is disclosed in which the
gold content is more completely extracted from refractory sulfidic
ores which contain large quantities of metal carbonates such as
dolomite than occurs in traditional gold extraction methods. The
process includes treating the ore in an autoclave at elevated
temperature and oxygen overpressure conditions in the presence of
soda ash or other alkaline materials. The autoclaving of the
slurried ore is performed for a sufficient amount of time to
oxidize the sulfidic compounds in the ore. The oxidized ore is then
more amenable to subsequent conventional gold extraction
techniques.
Inventors: |
Bakshani; Nandkumar (New
Orleans, LA), Yu; Peter H. (Harvey, LA) |
Assignee: |
Freeport Minerals Company (New
Orleans, LA)
|
Family
ID: |
25155571 |
Appl.
No.: |
06/792,025 |
Filed: |
October 28, 1985 |
Current U.S.
Class: |
423/29; 423/30;
423/31; 423/34 |
Current CPC
Class: |
C22B
11/08 (20130101); C22B 11/04 (20130101) |
Current International
Class: |
C22B
11/08 (20060101); C22B 11/00 (20060101); C22B
011/04 () |
Field of
Search: |
;75/11R,105,118R,106,108
;423/27,29,30,31,47,144,34 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Stoll; Robert L.
Attorney, Agent or Firm: Fisher, Christen & Sabol
Claims
We claim:
1. A pretreatment method for recovering gold comprising:
(a) forming an aqueous slurry of refractory sulfidic
gold-containing ore; and
(b) subjecting said aqueous slurry in the presence of an alkaline
material to an oxidation step, said oxidation step includes
simultaneously:
(i) autoclaving said aqueous slurry and
(ii) oxygenating said autoclaved aqueous slurry;
wherein said oxidation step is sufficient to oxidize refractory
sulfidic compounds in said ore and said autoclaving being at a
temperature of between about 400.degree. F. and about 500.degree.
F. and an oxygen overpressure of between about 50 psig and about
100 psig for an aqueous slurry residence time in said autoclave
sufficient to oxidize said refractory sulfidic compounds of said
ore.
2. The method according to claim 1, wherein said aqueous slurry
residence time in said autoclave is at least 2 hours.
3. The method according to claim 2, wherein a subsequent gold
extraction process is conducted, said subsequent gold extraction
process being a cyanide leach process.
4. The method according to claim 3 wherein said sulfidic compounds
of said ore are substantially pyrite compounds.
5. The method according to claim 4, wherein said ore contains at
least 8 percent dolomite.
6. The method according to claim 1, wherein said alkaline material
is soda ash, said soda ash being in a concentration of between
about 50 pounds and about 100 pounds per ton of dry ore.
7. A process for recovering gold comprising:
(a) forming an aqueous slurry of refractory sulfidic
gold-containing ore with an alkaline material;
(b) subjecting said aqueous slurry to an oxidation step, said
oxidation step includes simultaneously:
(i) autoclaving said aqueous slurry; and
(ii) oxygenating said autoclaved aqueous slurry;
(c) cooling said oxidized aqueous slurry to about ambient
temperature;
(d) adjusting said cooled aqueous slurry to about pH 10; and
(e) conducting a cyanide leach of said liquid fraction of said
aqueous slurry whereby said gold is separated from said aqueous
slurry;
wherein said autoclaving is at a temperature of between about
400.degree. F. and about 500.degree. F. and an oxygen overpressure
between about 50 psig and about 100 psig for an aqueous slurry
residence time in said autoclave sufficient to oxidize said
refractory sulfidic compounds of said ore.
8. The method according to claim 1, wherein said aqueous slurry
residence time in said autoclave is at least 2 hours.
9. The method according to claim 8, wherein said sulfidic compounds
of said ore are substantially pyrite compounds.
10. The method according to claim 9, wherein said ore contains at
least 8 percent dolomite.
11. The method according to claim 1, wherein said alkaline material
in soda ash, said soda ash being in a concentration of between
about 50 pounds and about 100 pounds per ton of dry ore.
12. A method for recovering gold from refractory sulfidic
gold-containing ore comprising:
(a) forming an aqueous slurry of said ore with a soluble alkaline
material;
(b) subjecting said aqueous slurry to oxidation in an autoclave,
said oxidation includes simultaneously:
(i) autoclaving said aqueous slurry, said autoclaving being at a
temperature between about 400.degree. F. and about 500.degree.
F.;
(ii) oxygenating said autoclaved aqueous slurry;
(c) cooling said oxidized aqueous slurry;
(d) thickening said aqueous slurry, said thickening includes
separating a liquid fraction from said aqueous slurry whereby a
solids fraction of said aqueous slurry having a solids
concentration of greater than about 40 percent by weight is
obtained;
(e) adding an alkaline agent in an amount sufficient to adjust said
cooled aqueous slurry to about pH 10;
(f) simultaneously contracting the oxidized aqueous slurry in a
plurality of stages with a cyanide complexing agent and a granular
activated carbon, said activated carbon being circulated
contercurrent to said cyanide aqueous slurry, a temperature of said
cyanide aqueous slurry being maintained between about 40.degree. F.
and 100.degree. F. whereby said gold is absorbed on said activated
carbon; and
(g) separating said gold laden activated carbon from said aqueous
slurry.
13. The method according to claim 12, wherein said alkaline
material is Na.sub.2 CO.sub.3.
14. The method according to claim 13, wherein said aqueous slurry
residence time in said autoclave is at least 2 hours.
15. The method according to claim 14, wherein said oxygenating in
said autoclave includes feeding an oxygen containing gas at an
overpressure between about 50 psig and 100 psig.
16. A method for recovering gold from gold containing ores
comprising:
(a) forming an aqueous slurry of refractory sulfidic
gold-containing ore having at least about 8 percent metal
carbonates, said aqueous slurry having a solids content between
about 20 percent and about 45 percent by weight;
(b) adding Na.sub.2 CO.sub.3 to said aqueous slurry at a rate of
between about 50 pounds and about 100 pounds of Na.sub.2 CO.sub.3
per ton of dry ore;
(c) autoclaving said formed aqueous slurry for an autoclave
residence time of at least 2 hours in an autoclave at a temperature
between about 400.degree. F. and about 500.degree. F. while
simultaneously oxygenating said slurry by feeding an oxygen
containing gas into said autoclave at an overpressure between about
50 psig and about 100 psig;
(d) cooling the oxidized aqueous slurry;
(e) thickening said aqueous slurry, said thickening includes
separating a liquid fraction from said aqueous slurry whereby a
solids fraction of said aqueous slurry having a solids
concentration of greater than about 40 percent by weight is
obtained;
(f) adding an alkaline agent in an amount sufficient to adjust said
cooled aqueous slurry to about pH 10; and
(g) recovering gold from said aqueous slurry utilizing a cyanide
leach technique.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a method for the recovery of
gold from refractory ores. More specifically, this invention
relates to a method for the recovery of gold from refractory
sulfidic ores which contain large quantities of metal carbonates
such as dolomite.
2. Prior Art
In recent years the mining industry has become increasingly
involved with developing methods for extracting gold from complex
refractory ores. This trend is due to the increasing scarcity of
free milling gold ore deposits. Gold ores are refractory when their
gold content is closely associated with metallic sulfides in the
ore. The metallic sulfides found in gold ore are substantially
pyrite and pyrite derivative compounds such as arsenopyrite.
Although sulfidic refractory ores have been known and studied for
years, the actual chemistry involved with their refractory nature
is not clearly understood. Additionally, sulfidic ores vary
considerably between deposits in both the refractory nature and the
mineral content of the ore. The mining industry has developed a
number of different processes to extract gold from sulfidic
ores.
U.S. Pat. No. 4,266,972 to Redondo-Abad et al. discloses a method
for treating a sulfidic ore containing precious metals and base
metals under an oxygen pressure of about 5 to 15 Kg/cm.sup.2 g (71
to 213 psig) at a temperature of 150.degree. C. to 250.degree. C.
The sulfidic ore is leached under these conditions by water to
dissolve zinc and copper, hydrolyze iron to hematite, and leave
lead and precious metals as insoluble sulfates. The lead, silver,
and gold are leached from a solid residue by a strong chloride
solution containing calcium chloride at a temperature in the range
of about 60.degree. C. to 90.degree. C., at a pH in the range of
about 7.0 to 1.0 and in the presence of about 1 gram per liter of
ferric chloride.
U.S. Pat. No. 4,431,614 to Makipirtti et al. discloses a method for
the separation of gold and silver from complex sulfide ore. The
method of this invention involves heating the sulfide ore at a
temperature of 600.degree. C. to 900.degree. C. at a sulfur
pressure of 0.2 to 1 atmosphere to bring the complex metal
compounds to a suitable form for a subsequent alkaline cyanide
leaching. The gold and silver is then dissolved by a cyanide
solution and separated from the insoluble residue. This method uses
a leaching time of about 8 hours and obtains yields of silver at
about 48 percent and yields of gold at about 96 percent.
U.S. Pat. No. 4,438,076 to Pietsch et al. discloses a method for
extracting gold and silver from an ore. This method involves a
process for leaching gold and silver from an ore in an alkaline
cyanide solution. The leaching is performed by maintaining the
slurry in a turbulent state at a pressure of 25 to 130 bar (363 to
1886 psig) while injecting oxygen into the slurry as the slurry
passes through a tube reactor in a continuous unidirectional flow.
The oxygen having a purity of at least 90 percent is injected such
that the CN/O.sub.2 molar ratio is at most 0.7 at 25 and 0.7 g/l
NaCN. The CN/O.sub.2 molar ratio is lower than 0.7 at pressures
higher than 25 bar. The slurry temperature is 70.degree. C. or less
during this reaction. Lime can be used to adjust the slurry pH. The
method requires a tube reactor which does not readily lend the
process to large scale industrial gold extraction operations.
U.S. Pat. No. 4,442,072 to Baglin et al. discloses a method for the
selective recovery of base metals and precious metals from ores.
The ores are smelted with a flux to form a matte and slag. The
matte is subsequently ground and leached with about 10 to 40 weight
percent sulfuric acid at a temperature of about 40.degree. C. to
about 100.degree. C. at atmospheric pressure. This step selectively
solubilizes nickel and iron in the ore after leaching. The ore is
roasted at about 300.degree. C. to 500.degree. C. and further
leached with dilute sulfuric acid at ambient temperature and
pressure to extract copper. The remaining residue has a high
concentration of platinum, palladium, and gold.
Extracting gold from refractory sulfidic ores is made more
difficult when the ores contain large amounts of basic metal
carbonates. One of the most common metal carbonates found in gold
ores is dolomite. Pressure leaching processes using acid as the
leaching agent have been used with refractory sulfidic ores. An
acid pretreatment step can be utilized when there is only a small
amount of dolomite in the ore.
U.S. Pat. No. 4,084,961 to Caldon discloses a method for the
treatment of metal bearing mineral material. This invention is a
pressure leaching process that can be preceded by an optional acid
pretreatment step wherein sulfuric and nitric acids are used to
remove unwanted metal carbonates from the ore. This reaction is
conducted under pressurized oxygen at elevated temperatures. Ores
having high concentrations of carbonates consume a correspondingly
higher amount of acid in pretreatment steps. This adds to the
overall expense of the gold leaching operation. In addition, the
use of corrosive acids in either a pretreatment step or in a
pressure leaching step can necessitate the use of expensive
corrosion resistant materials in the equipment in which the acid
treatment operation is conducted.
An example of an economical method for the recovery of gold from
refractory carbonaceous ores is U.S. Pat. No. 4,289,532 to Matson
et al. herein incorporated by reference. Refractory carbonaceous
ores owe their refractory nature to their carbon content rather
than to their sulfidic content as do the ores whose recovery is the
object of the present invention. This method subjects ores to
simultaneous cyanidation and countercurrent granular activated
carbon adsorption in two or more stages. Prior to cyanidation this
method includes an oxidation procedure, such as an oxygenation
and/or chlorination procedures to make the carbonaceous ore more
amenable to cyanidation. This method has been proven to be an
economical process for recovering gold from very refractory
carbonaceous ores, but provides less than desirable results with
refractory ores that possess both sulfidic and carbonaceous
characteristics.
The industry is lacking an effective, acid-free method of
recovering gold from sulfidic ores which are refractory to
conventional cyanidation techniques and which contain large
quantities of basic metal carbonates.
SUMMARY OF THE INVENTION
This invention is a method for recovering gold from a refractory
sulfidic gold-containing ore by oxidizing a slurry of the ore.
Oxygenation occurs in an autoclave at between about 400.degree. F.
and about 500.degree. F. with an oxygen overpressure of between
about 50 psig and about 100 psig for at least 2 hours. This
oxidation step occurs before a cyanide leach of the ore.
In typical gold extraction processes the method of this invention
includes grinding a refractory gold-containing ore having sulfidic
material such that 90 percent of the ore passes through a 100 U.S.
mesh screen. These ores typically contain at least about 8 percent
dolomite impurity. The ground ore is slurried with water to a
solids content of between about 20 and about 45 percent by weight.
Soda ash (Na.sub.2 CO.sub.3) is added at the rate of about 50 to
100 pounds per ton of dry ore. Lime (CaO) or caustic soda (NaOH)
can be used in place of soda ash. Soda ash is the preferred
alkaline agent because of its cost effectiveness when compared to
the other agents. The ore slurry is then fed to an autoclave
wherein the slurry is continuously agitated. Steam and oxygen are
fed to the autoclave such that the autoclave is operated at between
about 400.degree. F. and about 500.degree. F. with an oxygen
overpressure of between about 50 psig and about 100 psig. The
residence time per unit of volume of slurry in the autoclave is at
least 2 hours. Desirably the autoclave has baffles to increase the
turbulence and decrease "short circuiting" of the ore slurry flow
through the autoclave. The oxidized ore slurry exits the autoclave
and is ready for cyanide leaching. Optionally, a liquid-solid
separation step can be included to remove a portion of the slurry
water and consequently increase the percent of solids in the
slurry. This effectively increases the residence time of the solids
in the slurry during the cyanide leach step.
In the most desirable embodiments of this invention the cyanide
leaching is simultaneously performed with a carbon-in-pulp
adsorption step. The gold-loaded activated carbon is then separated
from the ore slurry and the gold is removed or stripped from the
activated carbon using standard techniques. The stripped gold can
be further refined.
BRIEF DESCRIPTION OF DRAWING
FIG. 1 is a flow diagram of the preferred embodiment of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
This invention is a method for recovering gold wherein an oxidizing
step is utilized before the traditional cyanide leach operation. In
the method of this invention a slurry of a refractory sulfidic
gold-containing ore is formed in the presence of an alkaline
material. The slurry is then subjected to a first oxidation step
which includes the simultaneous autoclaving and oxygenating of the
slurry. The slurry is then cooled to about ambient temperature. The
pH of the slurry is adjusted to about 10. Desirably, the cyanide
leach step involves the simultaneous complexing of gold in the
slurry with the cyanide ions and adsorbing the gold onto activated
carbon. In this manner, the gold-loaded activated carbon can be
separated or removed from the slurry.
The method of this invention is intended as a pretreatment
operation in which refractory sulfidic gold-containing ore is
conditioned for a subsequent conventional leach operation. The
invention is described herein with regard to its use in the cyanide
leach technique detailed in U.S. Pat. No. 4,289,532 to Matson. The
Matson process as discussed above subjects gold-containing ores to
two or more stages of simultaneous cyanidation and adsorption on
countercurrent flowing granular activated carbon. Other
conventional cyanide leach processes can be used with the
pretreatment method of this invention to make refractory ores
amenable to cyanide leaching. The pretreatment method of this
invention is not limited to any particular cyanide leach
technique.
Referring to FIG. 1, a sulfidic gold-containing ore 1 which is
refractory to conventional cyanidation techniques and which
contains greater than 8 percent dolomite by weight is fed through a
line 2 to a ball mill 5. The ball mill 5 also receives water 3 and
a recycled coarse fraction of the ore through a line 4 from a
cyclone separator 10. A slurry is thus produced in the ball mill 5
and contains about 70 percent solids. The ore is wet-ground in the
ball mill 5 such that 90 percent of the ore solids pass through a
100 U.S. mesh screen. The effluent slurry from the ball mill 5 is
passed through line 6 and collected in a sump 7 into which water 8
is added to adjust the slurry solids content to between about 35
and about 55 percent. The ore slurry is then fed through a line 9
to the cyclone separator 10. The cyclone separator 10 separates
fine and coarse ore particles. As already stated coarse ore
particles are recirculated from the cyclone separator 10 through a
line 4 back to the ball mill 5. Fine ore particles are slurried at
a solids content between about 20 to about 45 percent in the
slurry. The fine ore particles are passed from the cyclone
separator 10 through a line 11 to a mixing vessel 12 where an
alkaline agent 13, such as lime, caustic soda, or preferably soda
ash, is added to the ore slurry at the rate of about 50 to 100
pounds per ton of dry ore.
The ore slurry is then pumped through a line 14 into an autoclave
15 which is desirably a pressure vessel having internal baffles.
Steam 16 and oxygen 17 are also fed directly into the autoclave 15.
Other types of heating can be substituted for steam 16, however,
the design of commercial autoclaves makes steam injection the most
common or preferred heat source. Additional soda ash 18 can be
supplied to the autoclave 15 if necessary to regulate the pH of the
aqueous slurry. The autoclave 15 is operated at a temperature of
between about 400.degree. F. and about 500.degree. F. and an oxygen
overpressure of between about 50 psig and 100 psig. The autoclave
baffles serve to increase slurry turbulence. Increased slurry
turbulence within the autoclave improves the sulfidic compound
oxidation kinetics. The baffles also serve to prevent "short
circuiting" or backwashing of the ore slurry within the autoclave
15. The slurry enters at one end of the autoclave 15 and exits at
the other end. The baffles within the autoclave 15 direct the flow
of the slurry through the autoclave 15. The residence time of a
unit of volume of the ore slurry in the autoclave 15 is at least 2
hours and preferably 4 hours. The residence time must be sufficient
to allow the sulfidic compounds within the ore 1 to be oxidized.
Soda ash can be added into the autoclave. Typically, acid is formed
in the oxidation process within the autoclave 15 to cause the
slurry pH to be lowered to about pH 5.0.
The ore slurry exits the autoclave 15 by a line 19. The or slurry
is cooled by a conventional cooling vessel or equipment 20, that is
preferably conventional "flash cooling" equipment. The aqueous
slurry is then optionally transferred through line 21 to a
liquid-solid separator 22 to form a liquid fraction and a solids
fraction of the aqueous slurry. One type of solids separation
equipment suitable for this service is a "thickener". Excess water
is removed from the slurry via the liquid-solid separator and is
transferred through line 23 to the tailings pond 24. The purpose of
the liquid-solid separation step is twofold. Separation increases
the percent solids in the slurry and simultaneously reduces the
quantity of acidic slurry water requiring lime addition in a
subsequent conditioning procedure. The amount of lime addition
required during the conditioning procedure is, thus, slightly
reduced. The increase in the solids concentrate of the slurry
increases the residence time of ore particles in the cyanide leach
step. The liquid-solid separation step can be deleted when the
solids content of the ore exiting the autoclave is very high such
as greater than about 40 percent by weight of the slurry.
After exiting the liquid-solid separator 22 or, as discussed above,
the cooling vessel 20, the slurry is transferred through line 25 to
a conditioning tank 26 into which lime 28 is added to adjust the
slurry pH to about pH 10. Fresh water 27 can also be added to
replace at least some of the acidic water removed if the
liquid-solid separation step, discussed above, is performed. The
conditioned ore slurry leaves the conditioning tank 26 through line
29 and is passed to a gold recovery circuit utilizing conventional
cyanide leach techniques.
An essential feature of the present invention lies in the oxidizing
step carried out in the autoclave. For example, iron sulfide
(FeS.sub.2) in pyritic ores is oxidized to ferric oxide. The
oxidation of iron sulfide successfully reduces the refractory
nature of a sulfidic gold-containing ore. The reason for this
reduction in the refractory nature of the ore is not clearly
understood. For purposes of this invention the term, autoclaving,
represents a process that elevates the ore slurry temperature at a
sufficient pressure and for a sufficient time to sufficiently
oxidize a substantial amount of the sulfidic compounds in the ore
to make the ore amenable to subsequent gold extraction.
The overall chemical reaction involved in oxidizing refractory
sulfidic compounds of an ore is:
The formation of sulfuric acid in this reaction is responsible for
the drop in ore slurry pH as the ore slurry moves through the
autoclave.
The addition of soda ash or other alkaline agents to the ore slurry
in the autoclave appears to favorably increase the kinetics of the
oxidation reaction in the slurry. The addition of soda ash is
believed to be instrumental in increasing the rate of sulfur
oxidation through the overall reaction:
Any carbonaceous material contained in the slurry is also oxidized
through a similar overall reaction. Other alkaline agents such as
lime and caustic soda can be used in the autoclave. Soda ash has
proven the most compatible alkaline agent for this method of
autoclaving refractory sulfidic and carbonaceous gold-containing
ores.
The method of the present invention is preferably performed in a
continuous method for treating a refractory gold-containing ore.
One skilled in the art can use the invented method in a series of
slurry "batch" treating steps.
The disclosed process is intended for use on refractory sulfidic
ores containing large quantities of basic carbonates. This process
can also be effective on mixtures of refractory sulfidic and
carbonaceous ores. Sulfidic ores are generally much more refractory
than carbonaceous ores and mixtures of the two ores are normally
processed as if the entire ore mixture was composed of refractory
sulfidic ore.
EXAMPLES
The ore samples used in the following examples are obtained from an
ore deposit near Elko, Nev. A mineralogical analysis of the ore is
summarized in Table I below:
TABLE I ______________________________________ MINERALOGICAL
ANALYSIS Mineral Weight % ______________________________________
Illite/Muscovite 37 Quartz 30 Dolomite 19 Pyrite 9 Kaolinite 1
Hematite 1 Other 2 Gold 6 parts per million
______________________________________
The gold values of this ore were difficult to recover using
conventional cyanidation gold-recovery technology. When
conventional cyanidation procedures without preoxidation were
employed with this ore, gold recovery was about 20 percent. When
conventional cyanidation procedures with preoxidation of the
refractory ore material were employed, gold recovery was about 40
percent. These poor gold recoveries were attributed to the high
sulfide or pyrite content of the ore as shown in Table I.
The test procedure for these examples used batch tests. A sample of
ore was ground using a laboratory pulverizer such that 90 percent
to 100 percent of the ore passed through a 100 U.S. mesh screen.
Batches of 200 to 400 grams of the pulverized ore were slurried
with 600 to 800 milliliters of water and an alkaline agent such as
soda ash, lime, or caustic soda. The test slurry was placed in a
one-gallon capacity autoclave. The autoclave was heated by an
electrical heating mantle. When the desired autoclave temperature
was obtained, oxygen was injected into the autoclave until evidence
of pyrite oxidation was observed. Pyrite oxidation is exemplified
initially by a rising temperature and pressure in the autoclave. In
most of the examples a four hour retention or residence time of the
slurry in the autoclave was used. In certain examples residence
times of the slurry in the autoclave ranged from two to six
hours.
After completion of each autoclaving procedure, the heat source was
shut off and the autoclave was allowed to cool to ambient
temperature. Samples of the autoclaved slurry were then taken and
the pH of each sample was adjusted to between pH 10 and pH 10.5 by
adding lime. Sodium cyanide was added at a rate equivalent to 5
pounds per ton of ore solids. Westates brand activated carbon
(6.times.16 mesh size) was added at a concentration of 20 grams per
liter of slurry. Gold leaching and gold adsorption procedures were
then performed on the slurry using rolling bottles to agitate the
slurry. The slurries were agitated for 24 hours in the bottle-type
agitator rolls. Following the 24 hour agitation period, each bottle
was opened and the gold laden carbon was sieved or separated from
the slurry using a size number 48 U.S. mesh screen. The carbon-free
slurry was then filtered using a laboratory filter press to
separate the test slurries into solid ore and liquid fractions.
Both solid and liquid fractions were then analyzed for gold
content.
EXAMPLE 1
This example illustrates the improved gold yield from a refractory
sulfidic gold-containing ore using a pretreatment with soda ash and
an autoclave temperature and pressure sufficient to oxidize the
sulfidic compounds of the ore. In each test of this example, 200
grams of ore was crushed such that 90 percent to 100 percent passed
through a 100 U.S. mesh screen. To this ore 800 milliliters of
water having various doses of soda ash was added. The resulting ore
slurries were pretreated for 4 hours in an autoclave under an
oxygen overpressure of 100 psig. Individual tests were performed at
temperatures between 300.degree. F. to 500.degree. F. Two soda ash
concentrations were evaluated. These concentrations were equivalent
to 50 and 100 pounds of soda ash per ton of dry ore. The percentage
of gold extractions at the conditions tested are summarized in
Table II as follows.
TABLE II ______________________________________ PRETREATMENT WITH
SODA ASH Autoclave Gold Test Soda Ash Dosage Temperature Extraction
No. lbs Soda Ash/Ton Ore Solids .degree.F. %
______________________________________ 1 50 300 62 2 50 400 68 3 50
450 77 4 50 500 91 5 100 300 71 6 100 450 86 7 100 500 95
______________________________________
An autoclave temperature of 500.degree. F. was required in this
example to obtain a gold extraction in excess of 90 percent. A soda
ash concentration equivalent to 100 pounds per ton of ore solids
yielded slightly higher gold recoveries than did a soda ash
concentration equivalent to 50 pounds per ton of ore solids.
EXAMPLE 2
In this example test conditions were identical to those described
in Example 1 except that caustic soda was used as the alkaline
agent in place of soda ash. The soda ash concentrations and test
results are shown in Table III as follows:
TABLE III ______________________________________ PRETREATMENT WITH
CAUSTIC SODA Autoclave Gold Test Caustic Soda Dosage Temperature
Extraction No. lbs Soda Ash/Ton Ore Solids .degree.F. %
______________________________________ 1 50 300 37 2 50 400 68 3 50
500 92 4 100 300 58 5 100 400 80 6 100 500 93
______________________________________
As in Example 1 the percent of gold extraction achieved in this
example is directly related to both the autoclave temperature and
the alkaline material or caustic soda concentration. Of these two
variables, the autoclave temperature proved to have the most
significant effect on the percent of gold extraction.
EXAMPLE 3
This example illustrates the results obtained with the invented
method when lime is used as the alkaline agent. The autoclave test
conditions of this example included a temperature of 500.degree.
F., an oxygen overpressure of 100 psig, and a residence time for
the slurry in the autoclave of 4 hours. The percent of ore solids
for the test slurries is shown in Table IV as follows:
TABLE IV ______________________________________ PRETREATMENT WITH
LIME Reagent Dosage Gold Test Alkaline lbs Per Ton Ore Slurry
Extraction No. Agent Of Ore Solids % Solids %
______________________________________ 1 Lime 50 20 92 2 Lime 50 30
62 3 Lime 50 30 61 and Soda Ash 50
______________________________________
The test results of Table IV demonstrate that the use of lime as
the alkaline pretreatment agent is effective in slurries containing
20 percent ore solids, but relatively ineffective in slurries
containing 30 percent ore solids. This ineffectiveness in denser
slurries can limit the use of lime to less dense slurries. This
example demonstrates that the use of a lime and soda ash
combination is no more effective in gold extraction than is the use
of that amount of lime used without soda ash.
EXAMPLE 4
This example illustrates the influence of autoclave retention or
residence time on the percent of gold extraction. Retention time in
the autoclave was varied from between 2 to 6 hours. Three hundred
grams of the ground ore were slurried with 700 milliliters of
water. Soda ash was added at a rate equivalent to 100 pounds per
dry ton. The tests were performed at 500.degree. F. and 100 psig of
oxygen. The residence times used in this example and the respective
percents of gold extraction obtained at those residence times are
illustrated in Table V as follows:
TABLE V ______________________________________ EFFECT OF AUTOCLAVE
RETENTION TIME ON PERCENT GOLD EXTRACTION Autoclave Gold Retention
Extraction Time % ______________________________________ 2 80 4 90
6 94 ______________________________________
Table V illustrates that an autoclave retention time of 4 hours is
sufficient to achieve a gold extraction of 90 percent from the
refractory sulfidic gold-containing ore used in this example.
EXAMPLE 5
This example illustrates the effect of the use in the invented
method of various alkaline agents as possible alternatives to the
use of soda ash, caustic soda, and lime. The chemicals tested in
this example were ammonia and sodium hypochlorite. The autoclave
temperature was 500.degree. F. for the test and the oxygen
overpressure was 100 psig. A 4 hour residence time of the slurry in
the autoclave was used. The solids content of the test slurries was
30 percent. The alkaline agents and their concentrations in the ore
are listed in Table VI as follows:
TABLE VI ______________________________________ PRETREATMENT WITH
OTHER ALKALINE REAGENTS Ore Gold Test Reagent Dosage Slurry
Extraction No. Reagent lbs Per Ton of Ore % Solids %
______________________________________ 1 NH.sub.3 100 30 52 2 NaOCl
50 30 88 ______________________________________
The results of this example when compared to the results of the
other examples demonstrate that the use of ammonia in the
pretreatment oxidation method of this invention obtains a lower
percent of gold extraction than do other alkaline agents. This is
believed to be due to the turbulent conditions of the ore in the
autoclave causing the ammonia to foam. The foam generated in the
turbulent conditions of the autoclave causes a back-pressure in the
autoclave. This prevents the smooth entry of oxygen into the ore
slurry in the autoclave.
The use of sodium hypochlorite in this example resulted in a gold
extraction efficiency comparable to that of soda ash or caustic
soda. Sodium hypochlorite is more expensive than soda ash, caustic
soda, or lime, and is therefore not as cost effective as these
other alkaline agents.
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